Hydraulic brake systems for motor vehicles such as automobiles and light trucks are currently dual systems using two separate hydraulic circuits, usually for the front brakes and the rear brakes independently. Such systems utilize either a tandem master cylinder or dual master cylinders operated by a single pedal, and for this reason the pressure in both systems will be substantially identical under normal conditions, at least as far as these pressures can be measured at the master cylinder. With the two separate hydraulic brake circuits, which are hydraulically independent of each other and not interconnected, a hydraulic failure by leakage in one of the systems or circuits will not affect the other circuit, so that after a failure of either the front or the rear brakes, the other set will remain operative for stopping the vehicle, at least on an emergency basis.
With modern vehicles using power assists for the hydraulic brake circuits, a problem is presented that the brakes may be so powerful that a failure of one of the systems, so that only two wheels are braking, would not be noticed by the driver as the result of any substantially increased pedal effort, and therefore the vehicle might continue to be driven without operator knowledge of the failure, which would be an unsafe condition since there would therefore be no reserve braking for necessary minimum emergency stopping distances.
To overcome this problem, motor vehicles using these dual systems have incorporated a brake warning switch arrangement which will activate an indicator light on the dashboard of the vehicle in the event of a failure or inability to build up the required hydraulic pressure in one of the circuits. This has been accomplished by means of a pressure differential sensing switch, which may be integral with or separate from the master cylinder and consists of a body member having an axial bore in which a piston or spool member is mounted for axial sliding movement. Each end of the bore provides a trapped area which is is connected to one of the hydraulic brake circuits and, generally, the bores have equal area so that the spool or piston will be held in an intermediate or central position as long as the pressures at the ends are substantially equal. Generally, the spool may have a cam or ramp portion intermediate the ends which is utilized to actuate a switch mounted on the unit body so that whenever the pressure differential between the two hydraulic brake circuits exceeds a predetermined minimum, the spool will move in the direction of the lower pressure circuit to mechanically actuate an electrical switch to light a signal light on the dashboard of the vehicle. In order to ensure against operation unless the pressure differential exceed a predetermined minimum, there may be centering type biasing springs mounted in the bore to hold the piston in the middle position or, alternatively, various arrangements have been developed using differential areas and a floating sleeve so that a single spring can provide a centering action in both directions of movement of the spool.
However, it has been recognized that with such devices, problems may arise in view of the fact that the warning system must be operative at all times when the vehicle is in use, but may not be called upon to operate until many years after the vehicle has first been placed in use. It has been recognized that when the valve spool is perfectly balanced and does not have any bias to move, certain seals, such as O-rings and the like, may deteriorate either with age or because of contamination with the fluid, as well as because the sealing material takes a cold set under the environmental conditions which the unit experiences during operation of the motor vehicle. In such cases, if the seals fail, there may be a leakage from the system under normal operation which may even involve both hydraulic circuits or, alternatively, the seals could cause the unit to freeze so that the spool could not move even above the predetermined pressure levels in the event of an actual brake failure so that the driver would receive no warning and might continue to drive the vehicle with one defective brake system.